1. Field of the Invention
The present invention relates to inkjet printers, and more specifically, to a method for reducing thermal accumulation with inkjet printing through the use of sub-images.
2. Description of the Prior Art
Recently, the popularity of inkjet printers has increased dramatically due to their low cost and high quality. Since the price and quality are critical to the users' choices, printer vendors aggressively develop their products so that the products have lower cost and better quality so as to increase popularity and profits of their products. Therefore, developers are focusing on how to improve the performance of products under limited cost.
Most inkjet printers now use thermal inkjet printhead or piezo-electrical inkjet printhead to spray ink droplets onto a sheet of medium, such as paper, for printing. The thermal inkjet printhead includes ink, heating devices, and nozzles. The heating devices are to heat the ink to create bubbles until the bubbles expand enough to burst so that ink droplets are fired onto the sheet of paper through the nozzles and form dots or pixels on the sheet of paper. Varying the sizes and locations of the ink droplets can form different texts and graphics on a sheet of paper.
The quality of printing is closely related to the resolution provided by the printers, with higher resolutions requiring finer sizes of droplets. The size of the droplets is related to the cohesion of the ink. For instance, for droplets having identical amount of ink, ink with greater cohesion may have a smaller range of spread when they fall onto the paper, resulting in clearer and sharper printing. In the process of printing with the thermal inkjet technology, the heating elements of a printhead are activated to heat up the ink in the printhead for the creation of bubbles so that ink droplets are ejected from the nozzles onto a sheet of paper. As the temperature of the ink rises, the viscosity of the ink becomes lower. If the temperature of the ink is higher than a predetermined level, the viscosity of the ink could be abnormally low and ink droplets to be ejected would form larger dots onto the sheet of paper, resulting in a degraded quality of printing. Thus, the temperature control of the ink is a key to the improvement of the printing quality.
Please refer to FIG. 1. FIG. 1 shows a block diagram of a conventional inkjet printer 10. The inkjet printer 10 includes a central processing unit (CPU) 12, a printing controller 16, a printhead driver 18, and a printhead 20. During printing, data representative of images to be printed are fed into the inkjet printer 10. After processing of the data, the CPU 16 feeds image data 14 into the printing controller 16. The image data 14 includes information of locations, colors, and density of pixels corresponding to the images to be printed. In response to the image data 14, the printing controller 16 controls the printhead driver 18 and the printhead driver 18 causes the printhead 20 to print the images.
Please refer to FIG. 2. FIG. 2 gives an illustration of a portion of nozzles arranged on the printhead 20. For the sake of simplicity, the nozzles of the printhead 20 are represented as an array of nozzles 20′. The printhead 20 includes a plurality of nozzles and heating elements, and each of the heating elements is disposed in proximity to an associated nozzle to heat ink close to the nozzle for the ejection of ink droplets.
In the course of printing, a nozzle may eject ink droplets consecutively. The heat generated by the heating element associated with the nozzle may accumulate because consecutive triggering signals are applied to the heating element while there is no enough time for the heat produced to release completely. Besides, the ink temperature near the nozzle may also be greater than that near the other nozzles. If the heat accumulation is not well compensated, the ink temperatures near different nozzles will be different from each other. Because of the different temperatures, the ink near different nozzles will have different viscosity. The ink droplets ejected from different nozzles would be of different sizes, resulting in a degraded printing quality. Thus, temperature compensation is necessary for improving the printing quality of thermal inkjet printing.
Conventionally, there are two techniques for temperature compensation for use in inkjet printing apparatuses. In the first approach, temperature compensation is based on the temperature of the nozzles measured by a thermal resistor arranged near the nozzles. In addition, the temperature of the nozzles is determined by the variation of the resistance of the thermal resistor. However, the temperature obtained in this way is an average temperature of a part or all of the nozzles whereas the temperature of specific nozzles are unobtainable. In other words, if abnormal temperature increase is observed, it is still not possible to identify the specific nozzles that cause the temperature rise in such conventional approach. Therefore the temperature compensation actions taken may not be appropriate.
In the second approach, temperature compensation is based on predictions about heat accumulation while the predictions are made by analyzing pixels of the image desired to be printed. If the formation of the images on a sheet of printing medium requires the ejection of a large number of ink droplets corresponding to the pixels of the images, a high degree of heat accumulation is expected. Conversely, if the formation of the images on the sheet of printing medium requires the ejection of a small number of ink droplets corresponding to the pixels of the images, a low degree of heat accumulation is expected. During printing, in order to achieve temperature compensation, evaluation of energy applied to each of the nozzles is made in accordance with the predications about heat accumulation. However, during consecutive ejection of ink droplets, heat release of the nozzles is incomplete, and heat accumulation still occurs in each nozzle. Thus, the second approach is unable to effectively resolve the problem of heat accumulation in the nozzles.